FIELD OF THE INVENTION
The field of art to which this invention pertains is shrinkable materials and, more particularly, it is directed to processes for treating a tow of shrinkable fibers.
Specifically such invention is a process of treating a tow of aromatic polyamide fibers by moving the tow in folds through a chamber having a plug section which is operatively connected by a restricted opening to a treatment section and by heating the tow with steam in the treatment section.
Sufficient steam migrates into the plug section to cause shrinkage of the tow in this section. The cross-sectional area of the tow is substantially the same as the cross-sectional area of the entrance opening of the plug section as it enters such opening but it has a smaller cross-sectional area, due to shrinkage, as it leaves this section and moves through the restricted entrance opening of the treatment section. By making this opening smaller so that the cross-sectional area of the shrunken tow as it enters such restricted opening is substantially the same as the cross-sectional area of such opening, loss of steam through the entrance opening of the plug section is minimized.
More specifically, this invention is a process for dyeing a fiber structure of poly(meta-phenylene isophthalamide) fibers with a water-soluble dye by heating the amorphous, shrinkable fibers, as spun and prior to drying, with steam, in a specifically designed chamber, at a temperature from about 110.degree. C. to about 140.degree. C., and preferably at about 120.degree. C., for a time sufficient to diffuse substantially all of the dye into the minute pores in the fibers, throughout the fiber structure.
An organic water-insoluble material, such as an ultraviolet light screener, may also be mixed with the water-soluble dye and padded onto the water-swollen fibers prior to heating. While the dye is effectively diffused into the fiber structure at temperatures between 110.degree. C. and 140.degree. C., such structure must also be heated with steam at a sublimation temperature below the glass transition temperature of the fibers in order to sublime the screener into the pores of the fibers. The fibers are then, preferably, further heated with steam at about 165.degree. C. for a time sufficient to collapse the pores in the fibers and lock the dye therein. At this temperature the fibers also will crystallize and the fiber structure is thereby stabilized against progressive laundry shrinkage.